A rechargeable lithium battery has been recently paid more attention to as a power source for a portable small electronic device. It includes an organic electrolyte solution and thereby, has over twice as many discharge voltages as a conventional battery including an alkali aqueous solution, having high energy density.
A positive active material of a rechargeable lithium battery mainly includes an oxide of a transition element and lithium such as LiCoO2, LiMn2O4, LiNi1-xCoxO2 (0<x<1), and the like that can intercalate and deintercalate lithium.
A negative active material has included various-typed carbon-based materials such as artificial graphite, natural graphite, and hard carbon that can intercalate and deintercalate lithium. Since graphite among the carbon-based materials has low discharge voltage of −0.2V compared to lithium, and thereby, a battery including the graphite as a negative active material has high discharge voltage of 3.6, it can provide an advantage of energy density for a lithium battery. In addition, since it has remarkable reversibility, it can secure a long cycle-life for a rechargeable lithium battery. That is why it has been most widely used. However, when graphite is used as an active material to fabricate a substrate, it has a problem of lowering density of the substrate and thereby, lowering capacity in terms of energy density per unit volume. In addition, since it can negatively react with an organic electrolyte solution including graphite at high discharge voltage, a battery including it can get fired and blown off when the battery is mis-operated, over-charged, and the like.
In order to solve this problem, research on developing a new negative active material has been actively made. In particular, research on a negative active material for a rechargeable lithium battery with high-capacity has been made centering on metal materials such as Si, Sn, Al, and the like. When silicon (Si) reacts with lithium to form a compound, it can reversibly intercalate and deintercalate the lithium. It has theoretical maximum capacity of about 4020 mAh/g (9800 mAh/cc, gravity=2.23), which is much bigger than that of a carbon material. So, silicon is very promising as a negative electrode material with high capacity. However, since it has a big volume change due to reaction with lithium during the charge and discharge, silicon active material powder may be decomposed. In addition, the silicon active material powder may have a bad electrical contact with a current collector. This bad contact can sharply decrease battery capacity, leading to shorten cycle-life of a battery as the battery repeats cycles.
Accordingly, some suggestions have been made, which include forming a carbon layer on the surface of a silicon particle or using carbon composite formed by uniformly mixing silicon or a silicon-metal alloy with graphite. However, there is still requirement for development of a negative active material for a lithium rechargeable battery having high capacity and long cycle-life characteristics.